Heat treatment of steel in a protective atmosphere



March 30, 1954 M. J. sTU'rzMAN HEAT TREATMENT 0E STEEL 1N A PROTECTIVE ATMOSPHERE Filed Nov. 18. 1950 Figf VOLATILIZII.

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Patented Mar. 30, 1954 HEAT TREATMENT OF STEEL IN A PROTECTIVE ATMOSPHERE Milo J. Stutzman, Kansas City, Mo., assignor to Midwest Research Institute, Kansas City, Mo., a corporation of Missouri Application November 18, 1950, Serial No. 196,385

18 Claims. (Cl. 14S-16.5)

The present invention relates to a method for for heat treating steel in the presence of a gas atmosphere.

Protective gas atmospheres of controlled composition have been extensively used in the art of heat treating steel for two main purposes. The first of these is the carburization of low carbon content steels for the purpose of case hardening the steel surface. The second field in which controlled gas atmospheres are employed is in the heat treatment of relatively high carbon content steels to prevent oxidation and decarburization of the surface of the steel during heat treatment.

The mechanics of the carburization reaction involve two distinct phases. In the rst, carbon is absorbed from the gaseous atmosphere at the surface of the metal. in the next phase, the absorbed carbon migrates or is diffused into the interior of the steel. Through this reaction the compounds may also be introduced into the gaseous atmosphere so that nitriding of the steel occurs at the same time the carburization reaction is carried out.

The most commonly used gases for carburizing purposes are (1) natural gas, (2) natural gas mixed with air or partially burned gas, (3) butane, (4) propane or butane mixed with a gas obtained by cracking their vapors mixed with air. and (5) manufactured or city gas.

In using pure gases such as methane, ethane and butane, there is a tendency for the hydrocarbons to break down and form soot carbon which is deposited upon the steel articles as well as upon the furnace walls. This breakdown in-A terferes with the carburizing process, since austenitic steels do not absorb soot carbon at carburizing temperatures.

The presence of water vapor, if present in controlled amounts, aids in promoting the carburizaticn reaction. However, if an excess of water vapor is present, the atmosphere becomes slightly oxidizing, so that the surface of the steel is ozidized or decarburized at the temperatures commonly used in heat treating.

In order to secure efficient carburization in steel, it is necessary in conventional practice to balance all of the above considerations by a careful control of the carbon content of the carburizing gas. To accomplish the control of the carbon content in the carburizing gas requires' a careful control of the process in which the gas is manufactured, or recovered, and also requires large amounts of auxiliary equipment.

An object of the present invention is to provide a method for preparing a protective gas to be used in the heat treatment of steel articles, wherein the composition of the protective gas can be carefully controlled.

Another object of the present invention is to provide a protective atmosphere in which steel articles of high carbon content can be heat treated without appreciable oxidation or de-carburization.

I have now found that a very suitable gas atmosphere for carburizing or preventing oxidation oi steel can be produced by volatilizing a normally liquid composition containing as a major ingredient an organic compound having a high degree of solubility in water. Preferably, those organic compounds are employed which are miscible in water. At the temperatures employed for such heat treating processes, the organic compounds decompose into carbon, carbon monoxide, hydrogen, and other gases.

The preferred class of compounds within the practice of the present invention are the watermiscible aliphatic monohydric alcohols, such as methanol, ethanol and isopropanol. Besides the monohydric alcohols, compounds such as ethylene glycol, aldehydes such as acetaldehyde, ethers such as cli-methyl ether, acids such as acetic acid, and the like, may be employed but not necessarily with equivalent results. Water-miscible ketones such as acetone may be employed but are not as desirable as the monohydric alcohols as acetone has a tendency to form tar in the heat treating furnace and deposit soot upon the article and the walls of the furnace. As is well known, the organic compounds just described dene a group of compounds wherein the molecules of each contain only oxygen, carbon and hydrogen atoms, and all of these water-miscible compounds are known to decompose at the instant heat treating temperatures to form carbon, carbon monoxide and hydrogen, as the principal products of the composition.

The organic compounds of the type mentioned above form the major proportion of the gaseous mixture in the heat treating furnace. The percentage of the compound in the gaseous atmosphere will vary with the particular compound employed. If too little of the organic compound is present in the atmosphere, the carburization will be insufficient. If an excess of the organic compound is present, there will be a film of carbon deposited on the surface of the steel. As a general average, at least and preferably to 95% by volume of the mixture should consist of the organic compound, with the balance being essentially water. The appropriate concentration for certain monohydric alcohols are given below:

Percent Alcohol One of the advantages arising from the use of the compounds of the type mentioned over the uses of gases such as natural' gas and water gas is the fact that control of the carbon content of the carburizing atmosphere can be more easily attained. In the practice of the present invention, the organic compound and Water may be mixed in liquid phase in exactly the correct proportions and the mixture subsequently volatilized before introduction into the carburizing furnace. Thus, the carbonaceous content of the atmosphere, as well as the water vapor content, can be easily and carefully controlled to the optimum value for any specific type of steel Which is to be carburized.

In the carburizing process ofthe present invention, as in any carburizing process, several variables will affect the quality of the product. One of the more important variables is the control of gas pressure, as the rate of carburization at a given carburizing temperature and with any given carburizing gas increases substantially as the pressure increases.

Another variable which affects the carburzation is the rate of gas flow over the surface of the steel as the depth of the case produced may vary with an increase or de"rease of the rate of gas flow from the optimum value.

Another consideration is the condition of the surface of the steel article being carburized. The presence of mill scale, rust, grease or oil has a marked effect upon the rate of carburization.

The internal structure of the steel to be carburized is also an important consideration, as in many cases the work to be carburized has been forged or rough machined and frequently must be normalized to remove the eiects of working, refine the grain, and make the structure uniform to avoid warping during the carburizing and subsequent heat treatment.

Another important consideration is the carburizing temperature to be employed. While carburization will occur at relatively low temperatures on the order of 850 F, in practice the temperatures employed will normally be on the order of 1400 to 1800 F., and preferably from 1650 to 1'750 F. At temperatures below 1650 F., the ferrite of low carbon steels is not completely converted to austenite, so that the rate of carburization is too slow. Above temperatures of about 1750 F., grain growth is quite rapid, making subsequent working of the carburized article difcult.

The gas atmosphere may be introduced into the carburizing furnace in a number of manners. Four possible methods For introducing the oas into the furnace have been illustrated in the drawings, in which:

Figure l is a schematic representation of a system in which the ingredients making up the gas atmosphere are pre-mixed in liquid phase, volatilived and then introduced into the carburizing furnace;

Figure 2 is a schematic drawing of a similar system in which the ingredients of the atmosphere are separately volatilized before introduction into the furnace;

Figure 3 is a schematic drawing of a system in which the ingredients are introduced into the furnace in liquid phase and volatilized in a separate chamber in the furnace; and

Figure 4 is a schematic drawing of a system in which the ingredients are introduced in liquid phase to be volatilized by the heat of the furnace.

As shown on the drawings:

In the modification of the process of Figure 1, metered quantities of alcohol and water are introduced into a mixing chamber I Il where the liquids are mixed by suitable agitators. The rev sulting mixture, having a predetermined alcohol f the volatilizin;r chamber I2.

concentration, is then withdrawn from the mixing chamber I0 through a valved line II and introduced into a heated volatilizing chamber I2. The vapors of alcohol and Water leaving the volatilizing chamber I2 are next passed by means of a line I3 into a furnace I4 having refractory walls I5, the furnace I4 being heated to a carburizing temperature by heating means (not shown). It will be understood that the furnace I4 is usually purged with an oxygen-free gas prior to bringing the temperature up to the carburizing temperature to remove any oxygen present, or the furnace may be purged with the carburizing gas at heat treating temperatures with the furnace open. The steel articles to be carburized, in the illustrated instance, may be steel rods I3 which are disposed at the base of the furnace in contact with the vapor introduced from A vent Il is provided in the furnace I4 for disposing of gases leaving the furnace I 4. These gases may be condensed and recovered or may be burned if so desired.

The furnace arrangement illustrated in Figure 2 is identical with that of Figure l, except that the alcohol and water are separately volatilized for introduction into the furnace I4. A volatilizing chamber 2Q and a vapor conduit 2| are provided for introducing alcohol vapors into the furnace i4, and a corresponding volatilizing chamber 22 and conduit 23 are provided for introducing Water vapor into the furnace I4.

In the modification shown in Figure 3, the Water and alcohol are introduced as liquids through a pair of conduits 25 and 26. In this embodiment, the furnace I4 is provided with a vertical baffle 21 extending throughout the furnace, but terminating short of the top of the furnace, thereby providing a volatilizing chamber 28. As the alcohol and Water are introduced into the system, the hea-t of the furnace is suiiicient to rapidly volatilize the mixture so that the vapors pass over the baffle 27 and into contact with the steel articles I6. To prevent the vapors evolved in the vaporizing chamber 28 from being vented directly through the vent I1, the furnace is provided with a second baffle 30 which depends from the top wall and directs the vaporous mixture toward the base of the furnace.

In the embodiment of the invention shown in Figure 4, the water and alcohol are introduced as liouids into the top of the furnace by means of conduits 32 and 33. The heat of the furnace is, of course, sufcient to rapidly volatilize the liquid as it is introduced into the furnace to decompose the alcohol and provide a gaseous atmosphere. A vertical baille 35 is provided within the furnace to prevent the vaporized fluids from being vented directly through the vent I1.

invention.

While the drawings have been described in connection with a carburizing procedure in which alcohol and Water are used in the gas atmosphere, it will be appreciated that the systems therein shown are similarly applicable to other heat treating systems either of a carburizing or protective nature. In other words, the amount of organic compound used may range from that amount just sufficient to protect (i. e., prevent carburization and decarburization of) W carx bon steels to the substantially greater amount that is suiicient to effect carburization of high carbon steels; and the intermediate amounts are those amounts for protecting and for decarburizing high carbon Isteels and for carburizing low carbon steels. The amount of water used in each case is that amount at least suicient to prevent carbon deposition.

Table I shows the results of tests made on carburizing a mild steel test piece in the presence of various atmospheres.

In another series of tests, a drill rod having a carbon content of 0.95% was heat treated in the presence of various non-oxidizing atmospheres, with the results shown in Table II.

Tables I and II illustrate the importance of controlling the corcentr-ation of alcohol or other organic liquids present in the atmosphere, since if the percentage of alcohol is too high, carbon deposition appears on the surface of the article and along the Walls of the furnace. If the alcohol concentration is too low, the carburization lwill be incomplete. At the proper concentration for the particular article being treated, a hard case steel in the hypereutectoid range is produced 3 along the periphery of the article.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present Table I 6 i I 'claim as my invention:

l. The method of carburizing steel articles of relatively low carbon content which comprises heating said steel articles at a carburizing temperature in a heating zone, and introducing into said zone a mixture containing 5-20 volume of Water and 80-95 volume of a Water-miscible organic compound having only oxygen, carbon and hydrogen atoms in its molecule and capable of decomposing under the conditions in said heating zone into carbon and carbon-containing gases, the amount of Water being at least sucient to prevent carbon deposition and the amount of the organic compound being at least suicient to eect carhurization of the steel articles.

2. The method of carburizing steel articles of relatively low carbon content which comprises heating said steel articles at a carburizing temperature in a heating zone, and introducing into said zone a liquid mixture of 5-20 volume of Water and 80-95 volume of a Watermiscible organic compound having only oxygen, carbon and hydrogen atoms in its molecule and capable of decomposing under the conditions in said heating zone into carbon and carbon-containing gases, the amount of Water being at least suicient to prevent carbon deposition and the amount of the organic compound being at least suicient to eiTect carburization of the steel.

3. The method of carburizing steel articles of relatively low carbon content which comprises heating said steel articles at a carburizing temm perature in a heating zone and introducing into said zone a mixture consisting essentially of at least 5% by volume of water and at least 80% by volume of a Water-miscible organic compound having only oxygen, carbon and hydrogen atoms in its molecule and capable of decomposing under the conditions in said heating zone into carbon and carbon-containing gases, the amount of A. COLD-ROLLED MILD STEEL HEATED AT 1,600" F. IN VARIOUS ATMOSPHERES FOR 4 HOURS AND WATER-QUENCHED Carburized Sagpl Atmosphere Condition of Furnace gsb CasIe Iepth, Surface Conditions Grain nc es l 100% Isopropanol Carbon deposited 61 .0271 Black, carbon deposited Coarse 2 00% lsopropanol A little carbon 61 .0279. do Fine.

3 do do 61 0379.. .....do D0.

4 80% Isopropanol Very little carbon... 32 Dark oxide Do.

5 70% lsonronanol H2O condensed l2 Black, rough oxide scale Medium 6 95% Ethanol Cle 61 Blue-black, no scale ine.

7 85% Ethanol d 52 Gray-black, no scale.. Do.

8. do -63 Gray, no scale Medium Technical Methanol.

., OOLDj-ROLLED MILD STEEL HEATED A'T l,600 F. IN

FURNACE-COOL VEIOUS ATMOSPHEES lFOR 4 HOURS .AND

95% Ethanol Blue-black, no scale Isopropanol. Blue-black, to gray, spotted. Ethanol Black, no scale Technical Methancl Clean, one side black and the other side gray. 70% lsopropanol. Tar and water. No case.. Black, rough Table II C. DRILL ROD HEATED AT l,500 F. lN VARIOUS ATMOSPHERES FOR 1 HOUR AND WATER-QUENCHED Carburized Safrle Atmosphere Condition of Furnace lfs Caselglrsith, Surface Conditions Grain 95% Ethanol Carbon deposited 65 Blue-black no scale Very fine. Ethanol le 65 `Do. 85% Ethanol 63 F1ne. Technical Methano 63 Do. Acetone Tar. 65 D0. 70% Isopropanol Tar and H2O 62 Do.

`7 water being at least sufcient to prevent'carbon deposition.

4. The method of carburizing steel articles of relatively low carbon content which comprises maintaining said steel articles at a carburizing temperature in a heat treating zone and introducing into said zone a mixture containing at least 80% by Volume of a monohydric watermiscible alcohol and the balance essentially Water vapor in an amount at least suiiicient to prevent i carbon deposition.

5. The method of carburizing steel articles of relatively low carbon content which comprises maintaining said steel articles at a carburizing temperature in a heat treating zone, and introducing into said zone a mixture containing between 85% and 90% isopropanol by volume and the remainder Water.

6. The method of carburizing steel articles of relatively low carbon content which comprises maintaining said steel articles at a carburizing temperature in a heat treating zone, and introducing into said Zone a mixture containing between 90% and 95% ethanol by volume and the remainder Water.

7. The method of preventing surface oxidation and de-carburization of steel articles of relatively high carbon content during heat treatment which comprises heating said articles in a heat treating zone, and introducing into said zone a vaporized mixture of -20 volume of Water and 80-95 volume Aof water-miscible organic compound having only oxygen, carbon and hydrogen atoms in its molecule and capable of decomposing under the conditions in said heat treating zone into carbon and carbon-containing gases, the amount of Water being at least sufficient to prevent carbon deposition and the amount of the organic compound being at least suflicient to prevent decarburization of the steel the temperature in said heat treating zone being suiciently high to decompose the organic compound.

8. The method of preventing surface oxidation and de-carburization of steel articles of relatively high carbon content during heat treatment which comprises heating said articles in a v heat treatment zone, and introducing into said zone a mixture consisting essentially of 80-95 volume of a water-miscible monohydric alco- 9. The method of carburizing steel of rela tively low carbon content which comprises mixing a liquid water-miscible organic compound with water to form a mixture containing 5-20 volume of water and 80-95 volume of the organic compound, vaporizing the mixture and introducing the vaporixed mixture into a carburizing Zone in which the steel articles to be carburized are maintained at a carburizing temperature, the amount of water in said mixture being at least suincient to prevent carbon deposition in the carburizing zone and the amount of the organic compound being at least' suiiicient to effect carburization of the steel in the carburizing zone, and the organic compound having only oxygen, carbon and hydrogen atoms in its molecule.

10. The method oiA carburizing steel .0f a rela-` tively low carbon content which comprises mixing a liquid Water-miscible monohydric alcohol with water to form a mixture containing 5-20 volume of Water and 80-95 volume of the monohydric alcohol, vaporizing the mixture and introducing the vaporixed mixture into a carburizing zone in which the steel articles are maintained at a carburizing temperature, the amount of water in said mixture being at least suflicient to prevent carbon deposition in the carburizing zone and the amount of alcohol being at least suflicient to effect carburization of the steel in the carburizing zone.

11. The method of carburizing steel of a relatively loW carbon content which comprises heating said steel at a temperature between 1400* and 1800" F. in a heat treating zone, and introducing into said Zone a vaporized mixture of isopropanol, in an amount at least sufcient to effect carburzation of the steel and water, in an amount at least suf'licient to prevent carbon deposition in said zone, for a time suilicient to form a hypereutectoid case about the surface of the steel article, the volume ratio of isopropanol to Water being 85:15 to 90:10.

12. The method of carburizing steel of a relatively low carbon content which comprises heating said steel at a temperature between 1400 and 1800 F. in a heat treating zone, and introducing into said zone a mixture of ethanol, in an amount at least suflicient to effect carburization of the steel in said zone, and water, in an amount at least suiiicient to prevent carbon deposition in said zone, for a time suicient to form a hypereutectoid case on the surface of the steel article the volume ratio of ethanol to water being 90:10 to 95:5.

13. In a method of heat treating steel articles `in the presence of a protective atmosphere, the

steps of maintaining said heated steel articles during such heat treatment in a heat treating zone and introducing into said zone a liquid mixture of water and a water miscible organic compound having only oxygen, carbon and hydrogen atoms in its molecule, the amount of water being at least suflicient to prevent carbon deposition and the amount of the compound being at least suiiicient to effect carburization of steel of relatively low carbon content, the volume ratio of organic compound to Water being 80:20 to 95:5 and the temperature in said heat treating zone being sufliciently high to decompose the organic compound.

14. In a method of heat treating steel articles in the presence of a protective atmosphere, the

v,steps of maintaining said vheated steel articles during such heat treatment in a heat treating Zone, and introducing into said zone an mixture containing at least 20 volume per cent of Water and at least 80 volume of a water-miscible organic compound having only oxygen, carbon and hydrogen atoms in its molecule, the amount of water beingV at least sufficient to prevent carbon deposition and the amount of the organic water-miscible organic compound having only oxygen, carbon and hydrogen atoms in its molecule, the amount of water being at least sufficient to prevent carbon deposition and the amount of the organic compound being at least suiicient to prevent decarburization of the steel articles, and the temperature in the heat treating zone being suiicient to effect total decomposition of the organic compound.

16. In a method of heat treating steel articles of relatively high carbon content in the presence of a protective atmosphere, the steps of maintaining said heated steel articles during such heat treatment in a heat treating zone, and introducing into said zone a mixture consisting essentially of isopropanol, in an amount at least suilcient to prevent decarburization, and water, in an amount at least suficient to prevent carbon deposition, the isopropanol to water volume ratio being 85-90:15-10 and the temperature in the heat treating zone being sufficient to effect total decomposition of the isopropanol.

17. A method which comprises heating steel of relatively high carbon content at 1500 F. in a heat treating zone, admixing isopropanol and water in predetermined amounts within the volume ratio range of 85:15 to 90:10, vaporizing the isopropanol-water mixture, and introducing the vaporized mixture into the zone so as to effectively exclude from the zone all gases except the mixture and its decomposition products at the temperature of the zone, the amount of water in the mixture being at least sucient to prevent carbon deposition in the zone and the amount l0 of isopropanol being at least sufcient to prevent decarburization of the steel.

18. In a method of heat treating steel articles in the presence of a protective atmosphere, the steps of maintaining said heated steel articles during such heat treatment in a heat treating zone, and introducing into said zone a mixture of 'l5-90 Volume per cent of isopropanol and 25-10 volume per cent of Water, the amount of water being at least sufficient to prevent carbon deposition and the amount of isopropanol being at least suicient to prevent decarburization of the steel, and the temperature in the heating zone being sucient to decompose the isopropanol.

MILO J. STUTZMAN.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,516,645 Rennerfelt Nov. 25, 1924 1,768,317 Guthrie June 24, 1930 2,103,848 Harsch Dec. 28, 1937 2,329,896 Harsch Sept. 21, 1943 FOREIGN PATENTS Number Country Date 249,907 Great Britain Apr. 6, 1926 OTHER REFERENCES Carburizing, by American Society for Metals, pages 48, 50, 1938.

Metal Progress, June 1941, pages 693-696.

Controlled Atmospheres, by Jenkins, pages 379, 380, 1946. 

1. THE METHOD OF CARBURIZING STEEL ARTICLES OF RELATIVELY LOW CARBON CONTENT WHICH COMPRISES HEATING SAID STEEL ARTICLES AT A CARBURIZING TEMPERATURE IN A HEATING ZONE, AND INTRODUCING INTO SAID ZONE A MIXTURE CONTAINING 5-20 VOLUME % OF WATER AND 80-95 VOLUME % OF A WATER-MISCIBLE ORGANIC COMPOUND HAVING ONLY OXYGEN, CARBON AND HYDROGEN ATOMS IN ITS MOLECULE AND CAPABLE OF DECOMPOSING UNDER THE CONDITIONS IN SAID HEATING ZONE INTO CARBON AND CARBON-CONTAINING GASES, THE AMOUNT OF WATER BEING AT LEAST SUFFICIENT TO PREVENT CARBON DEPOSITION AND THE AMOUNT OF THE ORGANIC COMPOUND BEING AT LEAST SUFFICIENT TO EFFECT CARBURIZATION OF THE STEEL ARTICLES. 